1. Field of the Invention
The present invention relates generally to optical amplification suitable for optical fiber communication using wavelength division multiplexed signal light including a plurality of channels of optical carriers having different wavelengths, and more particularly to a method for such optical amplification and a system for carrying out the method.
2. Description of the Related Art
In recent years, a manufacturing technique and using technique for a low-loss (e.g., 0.2 dB/km) optical fiber have been established, and an optical communication system using the optical fiber as a transmission line has been put to practical use. Further, to compensate for losses in the optical fiber and thereby allow long-haul transmission, the use of an optical amplifier for amplifying signal light has been proposed or put to practical use.
An optical amplifier known in the art includes an optical amplifying medium to which signal light to be amplified is supplied and means for pumping the optical amplifying medium so that the optical amplifying medium provides a gain band including the wavelength of the signal light. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium and a pump light source for supplying pump light having a predetermined wavelength to the EDF. By preliminarily setting the wavelength of the pump light within a 0.98 .mu.m band or a 1.48 .mu.m band, a gain band including a wavelength of 1.55 .mu.m can be obtained. Further, another type optical amplifier having a semiconductor chip as the optical amplifying medium is also known. In this case, the pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are used. The plural optical carriers are individually modulated to thereby obtain a plurality of optical signals, which are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light, which is output to an optical fiber transmission line. On the receiving side, the WDM signal light received is separated into individual optical signals by an optical demultiplexer, and transmitted data is reproduced according to each optical signal. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case of incorporating an optical amplifier into a system adopting WDM, a transmission distance is limited by the wavelength dependence of gain which is represented by a gain tilt or gain deviation. For example, in an EDFA, it is known that a gain tilt is produced at wavelengths in the vicinity of 1.55 .mu.m, and this gain tilt varies with total input power of signal light and pump light power to the EDFA.
A gain equalization method is known as measures against the wavelength dependence of gain of an optical amplifier. This method will be described with reference to FIGS. 1 to 3.
FIG. 1 is a block diagram showing a conventional optical communication system adopting WDM. A plurality of optical signals having different wavelengths are output frm a plurality of optical senders (OS) 2 (#1 to #N), respectively, and next wavelength division mulitplexed in an optical multiplexer 4 to obtain WDM signal light. The WDM signal light is next output to an optical transmission line 6. The optical transmission line 6 is configured by providing a plurality of optical amplifiers 8 for compensating for losses and at least one gain equalizer 10 in an optical fiber transmission line 7. Each gain equalizer 10 may be provided by an optical filter. The WDM signal light transmitted by the optical transmission line 6 is separated into individual optical signals according to wavelengths by an optical demultiplexer 12, and these optical signals are next supplied to a plurality of optical receivers (OR) 14 (#1 to #N), respectively.
Referring to FIG. 2, there is shown an example of the spectrum of the WDM signal light output from the optical multiplexer 4 to the optical transmission line 6 in the system shown in FIG. 1. In FIG. 2, the vertical axis represents optical power, and the horizontal axis represents wavelength. In this example, the optical senders 2 (#1 to #N) output optical signals having wavelengths (.lambda..sub.1 to .lambda..sub.N), respectively. When preemphasis is not considered, the optical powers of the optical signals in all the channels are equal to each other in general. In this example, the band of the WDM signal light is defined by the wavelength range of .lambda..sub.1 to .lambda..sub.N as shown by reference numeral 16.
If each optical amplifier 8 in the system shown in FIG. 1 has a wavelength dependence of gain in the band 16 of the WDM signal light, the wavelength dependence of gain is accumulated over the length of the optical transmission line 6, causing an interchannel deviation in signal power or signal-to-noise ratio (optical SNR). In the gain equalization method, the characteristics of each gain equalizer 10 are set so as to cancel the accumulated wavelength dependence of gain of the optical amplifiers 8. This will now be described more specifically with reference to FIG. 3.
In FIG. 3, the broken line shown by reference numeral 18 represents the accumulated wavelength dependence of gain of the optical amplifiers 8, and the solid line shown by reference numeral 20 represents the wavelength dependence of loss in the gain equalizer 10. In the example shown, the wavelength dependence of gain is canceled by the wavelength dependence of loss in the band 16 of the WDM signal light, thereby achieving gain equalization in the whole of the optical transmission line 6.
In the case that an EDFA is used as each optical amplifier 8, the wavelength dependence of gain of the EDFA is asymmetrical with respect to a wavelength axis in general. In contrast, the wavelength dependence of loss of one optical filter usable as an element of each gain equalizer 10 is symmetrical with respect to a wavelength axis in general. Accordingly, in the case that each gain equalizer 10 includes only one optical filter, the asymmetrical accumulated wavelength dependence of gain of the optical amplifiers 8 cannot be compensated. As the optical filter, a dielectric multilayer film filter, etalon filter, Mach-Zehnder filter, etc. are known. These filters can be precisely manufactured, and the reliability has been ensured.
As the related prior art to compensate for the asymmetrical wavelength dependence of an optical amplifier, it has been proposed to configure a gain equalizer by combining two or more optical filters having different wavelength dependences of loss. With this configuration, the wavelength dependence of gain can be canceled by the wavelength dependence of loss with high accuracy in a given band of WDM signal light.
Additional details of the gain equalization method is described in Reference (1) shown below, and additional details of the combination of plural optical filters is described in References (2), (3), and (4) shown below.
(1) N. S. Bergano et al., "Wavelength division multiplexing in long-haul transmission systems", JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 14, NO. 6, JUNE 1996, pp1229-1308.
(2) K. Oda et al., "128-channel, 480-km FSK-DD transmission experiment using 0.98 .mu.m pumped erbium doped fibre amplifiers and a tunable gain equaliser", ELECTRONICS LETTERS, Jun. 9th 1994, Vol. 30, No. 12, pp982-983.
(3) T. Naito et al., "85-Gb/s WDM transmission experiment over 7931 km using gain equalization to compensate for asymmetry in EDFA gain characteristics", First Optoelectronics and Communications Conference (OECC '96) Technical Digest, July 1996, PD1-2.
(4) T. Oguma et al., "Optical gain equalizer for optical fiber amplifier", Communications Society Conference, IEICE, 1996, B-1093 (pp578).
Referring to FIGS. 4A and 4B, there are shown examples of an optical spectrum after transmission in a system adopting the gain equalization method. In each example, a plurality of steep signal spectra are superimposed on a relatively gentle noise spectrum. In the example shown in FIG. 4A, a deviation in signal power is suppressed by gain equalization. That is, gain equalization is performed so that the peaks of optical powers of the signal spectra are equal to each other. In this case, a signal-to-noise ratio (optical SNR) given as the length of each signal spectrum on the basis of the noise spectrum differs according to channel, that is, there remains an interchannel deviation in optical SNR. In a land communication system, for example, gain equalization is performed in such a manner that the interchannel deviation in optical SNR is permitted and the interchannel deviation in signal power is eliminated. Conversely, gain equalization may be performed in such a manner that the interchannel deviation in signal power is permitted and the interchannel deviation in optical SNR is suppressed as shown in FIG. 4B.
In either case of the conventional gain equalization method, attention is paid to only one of the signal power and the optical SNR, and gain equalization is performed on either subject. As a result, the conventional gain equalization method has a problem caused by a deviation remaining in the other of the signal power and the optical SNR. For example, in the case of performing gain equalization on the optical SNR deviation as shown in FIG. 4B, a deviation remains in the signal power, causing a problem in system construction such that a signal level diagram at a receiving terminal station differs. In the case of performing gain equalization on the signal power deviation as shown in FIG. 4A, a deviation remains in the optical SNR, causing a problem such that a transmission quality varies according to channel.